Screening methods for the rapid determination of Y chromosomal ploidy (peripheral blood leukocyte fluorescence analysis) are being applied to male participants in the national Health and Nutrition Examination Survey (HANES). Analyses are thus far completed on 4642 males in this population, with the finding of five individuals who may have a 47, XYY chromosome constitution. Further studies by different cytologic methods are being applied to these five suspect slides in order to ascertain those with a high degree of probability for the YY karyotype. The physical and psychosocial characteristics of individuals with the YY karyotype will be studied, by means of the other data accumulated on these subjects by the HANES. The present study will thus both determine the prevalence of YY aneuploidy in a general, ambulatory population of adult males and point out any medical or other concomitants that may be causally related to this karyotype. A further 2000 individuals will be screened for Y chromosomal ploidy in the coming year.